1. Field of the Invention
The present invention relates to a communication terminal device. More particularly, the present invention relates to a communication terminal device adapted to the physical characteristics of a telecommunication network and a method for adapting the communication terminal device to the physical characteristics of the telecommunication network.
2. Description of the Related Art
A communication terminal device, which transmits and receives voice signals or facsimile signals, which is connected to a telecommunication network such as a Public Switched Telephone Network (PSTN) has been widely used. Telecommunication terminal devices embedding various functions are also widely used. Particularly, facsimile devices, which include a facsimile unit to embody facsimile functions and a telephone circuit unit to embody telephone functions, respectively, are widely used.
FIG. 1 is a block diagram for depicting operation of a facsimile device having telephone functions according to the conventional art.
The communication terminal device in FIG. 1 includes a modular jack 10 for connecting to a telecommunication network, a facsimile unit 20, a telephone circuit unit 30, and a switch 15 for connecting to a telephone network, such as a public switched telephone network (PSTN), to the facsimile unit 20 or the telephone circuit unit 30. The PSTN is connected to the communication terminal device through the modular jack 10. When the PSTN is connected to the telephone circuit unit 30 by the switch 15, a loop current of the PSTN flows into the telephone circuit unit 30, and telephone functions are activated. Meanwhile, the facsimile unit 20 is disconnected from PSTN.
A bridge diode (BD) 35 steadily supplies a voltage of a uniform polarity to the telephone circuit unit, regardless of the voltage polarity of the signals from PSTN. The reason that a voltage of uniform polarity has to be supplied steadily to a telephone circuit unit 30 is to keep biasing the transistor elements constituting the telephone circuit unit 30.
Each of the telephone circuit unit 30 and the facsimile unit 20 further includes a DC feeding circuit (not shown) for supplying a DC voltage. The DC feeding circuit maintains the telephone line connection by continuously supplying a DC voltage to keep the connection between the communication terminal device and PSTN from being disconnected. A ring detector 25 delivers ring signals incoming from the PSTN to a Central Processing Unit (CPU) 24, and notifies the CPU 24 of calls from the PSTN. After determining that there is a ring signal, the CPU 24 distinguishes whether the signal delivered from the PSTN is a telephone signal or a facsimile signal, and then conveys it to the respective components.
The telephone circuit unit 34 processes voice signals incoming from the PSTN and delivers them to a handset 36. The telephone circuit 34 also processes the voice signals, which are input from the handset 36, and delivers them to the PSTN. And, the telephone circuit 34 controls the magnitudes of the side-tone signals, which are fed back to the handset 36 among the signals input from the handset 36 and transmitted to the PSTN.
A modem 26 decodes the facsimile signals delivered from the PSTN. An impedance matching unit 28 includes a transformer element and a capacitor to match the impedances of the PSTN and the modem 26. Therefore, when the PSTN is connected to the facsimile unit, it enables the facsimile signals and the like to be transmitted and received between the PSTN and the modem 26. The capacitor included in the impedance matching unit 28 prevents the transformer element from being damaged by an excessive DC current and transmits AC signals only.
As integrated circuit techniques develop, the facsimile unit, the telephone circuit unit, and the like, which are mentioned above, can be integrated in a single element. The telephone circuit unit offered in a single element form includes a microphone terminal for receiving voice signals from the handset and a receiving terminal for receiving voice signals from the PSTN. As noted above, a part of the voice signals, which is received from the microphone terminal and transmitted to the PSTN, is fed back to be delivered again to the handset by the receiving terminal.
The physical characteristics of the PSTN in respective countries all over the world are different. Therefore, each country's PSTN requires a communication terminal device to meet a certain specification, which matches the terminal device to the physical characteristics of the local PSTN. The physical characteristics of public telephone networks of each nation are, for example, the magnitude and frequency characteristics of the voice signals transmitted from the handset to the PSTN, the magnitude and frequency characteristics signals of voice signals, and the magnitude of side-tone signals delivered from PSTN to the handset.
Since these specifications are different in every country, the specifications of the communication terminal devices such as telephone sets must be set differently, corresponding to each and every nation where the devices are exported. That is, in order to export the communication terminal devices according to the conventional art, circuit element values are set differently for each respective country to constitute a separate telephone circuit unit satisfying the requirements related to the telephone characteristics of each country. Although the communication terminal device has the same functions, different telephone circuit units must be provided to let the telephone set operate in each country. Therefore, production costs rise. Furthermore, when a malfunction of the communication terminal device occurs in a certain country, standardized repair service after sales is impossible since a repair is only possible after understanding each nations' specification for the telephone circuit unit.
Therefore, using the unified telephone circuit unit, a communication terminal device adapted to the physical characteristics of the different telecommunication networks in all nations is required.